The present invention deals with a two-stage cooling system primarily of the type used in fruit or other food storage systems. One of the primary advantages to using a two-stage system lies in the fact that the primary system may utilize a more efficient, yet caustic, refrigerant within the system and the secondary, closed loop system which extends into the storage area uses a relatively safe refrigerant. If there is a leak within the storage area extensive damage to the food product will not result.
As will be immediately obvious, one of the major concerns with food storage is maintaining a proper temperature within the storage area. Prior art systems, some of which will be described in detail hereinafter, have utilized two-stage systems wherein the control of the temperature within the storage area is regulated by a control system within the secondary system.
One of the problems with the two-stage cooling system for food products, although enabling the food product to be primarily separated from the caustic refrigerant, includes the possibility of leakage within the system thus allowing the caustic primary refrigerant to enter into the secondary system, greatly endangering the food products. The secondary system may well not be of a material resistant to the caustic, primary refrigerant.
Yet another problem with the utilization of a two-stage system wherein the second stage is not sufficiently protected from the caustic refrigerant of the primary stage lies in the fact that the tubing or coils as well as other portions of the system must be of a material which will withstand the caustic material in order to prevent corrosion and leakage, thus greatly increasing the cost.
Prior art references known to the inventor dealing with a two-stage refrigeration system include U.S. Pat. No. 1,996,441 granted to Smith on Apr. 2, 1935 which discloses a two or more stage system having substantially uniform pressure within the circuits and control means within the secondary system.
U.S. Pat. No. 2,022,764 granted to Gibson et al. on Dec. 3, 1935 deals with a two-stage refrigeration apparatus wherein the temperature control is depended upon the pressure and temperature within the secondary loop.
U.S. Pat. No. 2,986,903 Cocher et al. deals with a particular heat exchanger for use in a system which includes two-stage refrigeration.
U.S. Pat. No. 3,363,430 granted to White on Jan. 16, 1968 deals with a two-stage condensing loop utilizing different refrigerants but providing no indication as to the means of control of temperature.
U.S. Pat. No. 3,683,640 granted to Eber discloses a refrigeration apparatus having a secondary refrigeration system.
U.S. Pat. No. 3,985,182 granted to Hara et al. discloses a heat transfer device in which there is provided a closed loop consisting of a descending flow tube for liquid and an ascending flow tube for water.
U.S. Pat. No. 4,025,326 granted to Leonard discloses a refrigeration unit having a closed secondary system connected in a heat exchange relationship with the primary system. In this system, the secondary system is a heating rather than a cooling system.
With the above known prior art and problems in mind, it is an object of the present invention to provide a two-stage cooling system wherein the refrigerant of the second system is contained within a closed loop utilizing a far less caustic refrigerant and thus permitting the utilization of less expensive conduits and the like.
It is another object of the present invention to provide a two-stage cooling system wherein the heat transfer between the two systems is efficient enough that the control for the temperature is located only in the primary system and provides adequate sensitivity in both systems.
It is still another object of the present invention to provide a two-stage cooling system wherein there is a differential in pressure between the primary caustic system and the secondary inert system whereby if there is a leak the flow would be from the inert to the caustic thereby preventing the introduction of caustic material into the secondary system.
Yet another object of the present invention is to provide a warning system which will prevent further operation upon the happening of an endangering condition.